In a full-duplex hands-free telephone or speaker phone, sound uttered by a first person calling to the speaker phone and emanating from the speaker are picked up by the microphone which is intended to pick up sounds uttered by the second person who is the user of the speaker phone. Careful design of the hands-free telephone can prevent positive feedback which results in howling. A second form of feedback can be present in which the first speaker's words are transmitted back to him or her delayed in time at a reduced magnitude and often distorted. This echo of one's own words is very distracting to most users and is normally controlled by the use of an acoustic echo canceller circuit. If the hands-free telephone is a cellular telephone in an automobile, adaptive noise suppression is required because the signal-to-noise ratio is low and the characteristic of the noise varies with time.
In known hands-free digital cellular telephones, the acoustic echo cancellation and noise suppression problems have been addressed as separate problems. Therefore, applying these solutions together may result in an inefficient system. For example, the full-band LMS algorithm disclosed in Sondhi, M. M. and Kellerman, W., Adaptive Echo Cancellation for Speech Signals, Chapter 11 from Advanced in Speech Signal Processing, Ed. by S. Furui and M. M. Sondhi, Marcel Dekker, 1991, can be used to provide an effective and straightforward solution to echo cancellation. Recently, researchers have been using a sub-band acoustic echo canceller because this requires less computational complexity and provides faster convergence to the filter coefficients that produce optimum echo cancellation. The noise suppression algorithm is commonly based upon the spectral subtraction method. In the spectral subtraction method, for the noise-only period, the noise spectrum is estimated using Fast Fourier Transform (FFT) or band pass filter banks. When the speech signal with noise comes in, the noise spectrum estimate is subtracted from the noise signal spectrum. The spectral subtraction method performs well for enhancing the signal-to-noise ratio but may create an artifact called "musical noise". A system which utilizes a smoothed spectrum for spectral subtraction in order to avoid the production of musical noise is shown in the U.S. patent application Ser. No. 08/426,746 filed Apr. 19, 1995 by Allen V. McCree and assigned to Texas Instruments Incorporated, which is incorporated herein by reference.
FIG. 1 shows a prior art system in which the two algorithms, one for the acoustic echo cancellation and one for noise suppression are applied independent of each other. In the system shown in FIG. 1, acoustic echo cancellation is provided by block 110 and noise suppression is provided by block 134. The output signal on line 104 used to drive speaker 106 is provided by amplifier 102 from well-known circuitry inside the hands-free telephone (not shown). The output signal on line 104 is also provided by line 108 to an analysis filter 112 which generates a sub-band signal on line 114 at the output of the analysis filter. The sub-band signal on line 114 is provided to adaptive filter 116. The output of adaptive filter 116 on line 118 is provided to summing amplifier 120 which also receives an input from analysis filter 122. Analysis filter 122 provides a sub-band signal from the input of microphone 124 which is the acoustic sounds emanating from the user also known as "near-end speech". The output of summation amplifier 120 on line 126 is provided to synthesis filter 130 which generates a full-band signal on line 132 and also via line 128 to the adaptive filter 116 to adjust the coefficients utilized by the filter 116. The coefficients of the adaptive filter 116 are adjusted in order to provide acoustic echo cancellation, as is well known in the art. Block 110 shows a sub-band acoustic echo cancellation system. If the system were to be implemented in the full-band domain, analysis filters 112 and 122 and synthesis filter 130 would be omitted.
The full-band reduced echo signal on line 132 is input to noise suppression circuit 134 at FFT generator 136. The output of FFT generator on line 138 is input to noise suppression circuit 140 which performs the spectral subtraction. The output of the spectral subtraction circuit on line 142 is fed into Inverse Fast Fourier Transform (IFFT) circuit 144 which produces the full-band signal on line 146 as an output of the acoustic echo cancellation and noise suppression circuit 100.
The echo cancellation algorithm may cancel the echo signal by 25 dB, for example, but the remaining echo is still able to be heard. Therefore, a residual echo suppression circuit may be used to repress the remaining echo signal more completely. However, prior art residual echo suppression circuits attenuate the signal when a detection circuit detects a remaining echo signal. This method, however, may produce the so-called "switching" effect, when the signal to noise ratio is low. The switching effect is caused by the antenuator suppressing the noise components all through the period when the residual echo signal should be suppressed, which in turn creates abrupt changes in the background noise. Accordingly, there is a need for echo suppression without creating the switching effect.